1. Technical Field
The invention relates to lasers. More particularly, the invention relates to an upconversion laser for the generation of blue light.
2. Description of the Prior Art
Generation of laser light at blue visible wavelengths is of interest in disk memory, large displays, and biosciences. The wavelength regions in the green and blue have been difficult to attain with semiconductor lasers though ongoing research is being performed in the field. Visible blue lasers have been achieved with Argon-Ion gas lasers. These lasers consume large amounts of power, are relatively expensive, and need special cooling requirements which make them unwieldy for instrumentation applications such as in the biosciences. Efforts are also being directed at blue-light generation using doubling structures and direct gap semiconductor lasers.
Infrared-pumped upconversion lasers in glass fiber are potentially compact sources for visible CW radiation. Blue, green, orange, and red upconversion laser oscillation has been demonstrated in Pr.sup.3+ doped fluoride fiber pumped by two Ti:sapphire lasers operating at 835 and 1010 nm (see, for example R. G. Smart et al, CW room temperature upconversion lasing at blue, green and red wavelengths in infrared-pumped Pr.sup.3+ -doped fluoride fibre, Electron. Lett., vol. 27, pp. 1307-1309 (1991)). See, also J. Y. Allain et al, Blue upconversion fluorozirconate fibre laser, Electron. Lett., vol. 26, pp. 166-168, (1990).
Using a Ti:sapphire laser operating near 850 nm as a pump source, Allain et al (Electron. Lett., vol. 27, pp. 1156-1157 (1991)) were able to generate 635 nm laser light from Yb.sup.3+ /Pr.sup.3+ -codoped fluoride fiber. Later, using two semiconductor laser diode pumps, one operating at 985 nm or 1016 nm, the other at 833 nm, Piehler et al (Electron. Lett., vol. 29, No. 21, pp. 1857-1858 (1993), were able to obtain upconversion lasing at 521 and 635 nm. A laser design for 450 nm light generation using upconversion lasing in trivalent Thulium-doped-fluoride glass has also been previously disclosed (see, for example Emmanuel W. J. L. Oomens, Device for Generating Blue Laser Light, U.S. Pat. No. 5,067,134, 19 Nov. 1991).
FIG. 1 is an energy level diagram showing a pumping method and transitions for a Pr.sup.3+ fluoride fiber upconversion laser. Dual-wavelength pumping or single wavelength pumping can be used in such laser. Because the lifetimes of the energy states become relatively long-lived in a fluoride glass host, absorption of pump photons is possible from various intermediate energy levels between the desired .sup.3 P.sub.0 state (39 .mu.s) and the ground state (see, for example D. Piehler et al, Laser-diode-pumped red and green upconversion fibre lasers, Electron. Lett., vol. 29, No. 21, pp. 1857-1858 (1993)). Thus, it is easily imagined that one pump photon 12 could create GSA (ground state absorption) to the .sup.1 G.sub.4 (100 .mu.s) level and another pump photon 16 could boost the ion from that energy level or some decayed level above the ground state to the desired .sup.3 P.sub.0 level. The excited ion 14 in this state has favorable branching to the ground state which is used as a lasing transition.
Because there are multiple transitions, multiple pump combinations could be envisioned to achieve the desired pumping of the .sup.3 P.sub.0 (blue) level. Published reports have shown simultaneous pumping at 835 nm and 1017 nm can sufficiently invert the blue level in a fluoride glass to achieve lasing (see, for example Y. Zhao et al, Efficient blue Pr.sup.3+ -doped fluoride fibre upconversion laser, Electron. Lett., vol. 30, pp. 967-968 (1994)). Using Ti-sapphire lasers operating at 1010 nm and 835 nm, researchers have generated blue laser light at 491 nm from Pr-doped fluoride fiber.
The use of a trivalent ytterbium sensitizer ion for populating the Pr.sup.3+ -&gt;.sup.1 G.sub.4 level (see FIG. 2) has been proposed for application to 1.3 .mu.m amplification (see J. Y. Allain et al, Energy transfer in Pr.sup.3+ /Yb.sup.3+ -doped fluorozirconate fibres, Electron. Lett., vol. 27, pp. 1012-1014, (1991)), and upconversion lasers (see J. Y. Allain et al, Red upconversion Yb-sensitized Pr fluoride fibre laser pumped in the 0.8 .mu.m region, Electron. Lett., vol. 27, pp. 1156-1157 (1991)). The advantages of codoping with Yb.sup.3+ are three-fold. The broad absorption band permits a wide pump wavelength range for activating the Yb.sup.3+ ion and cross-relaxation to other ions. Strong absorption permits shorter lengths of fluoride fiber that still yield adequate pump absorption. The simple energy structure of the Yb.sup.3+ ion reduces the possibilities for backwards energy transfer from the activator ion to the Yb.sup.3+ sensitizer ion, thus limiting the lifetime reduction of the 3P0 metastable state.
The addition of a sensitizer such as Yb.sup.3+ has also been reported (see, for example J. Y. Allain et al, Red upconversion Yb-sensitized Pr fluoride fibre laser pumped in the 0.8 .mu.m region, Electron. Lett., vol. 27, pp. 1156-1157 (1991); Y. Ohishi, Gain characteristics of Pr.sup.3+ -Yb.sup.3+ codoped fluoride fiber for 1.3 .mu.m amplification, IEEE Photon Technol. Lett., vol. 3. No. 11, pp. 990-992 (1991); and J. Y. Allain et al, Energy transfer in Pr.sup.3+ /Yb.sup.3+ -doped fluorozirconate fibres, Electron. Lett., vol. 27, pp. 1012-1014, (1991)).
With a sensitizer, the range of wavelengths over which the Pr.sup.3+ ion can be activated is extended due to a process called cross-relaxation, where energy is transferred from the Yb.sup.3+ ion residing in the 2F5/2 state and the .sup.1 G.sub.4 state of the Pr.sup.3+ ion. This allows for great flexibility in choosing laser pump wavelengths. Blue light generation has been demonstrated using a Ti-sapphire laser operating at 850 nm to a pump Yb.sup.3+ sensitized Pr.sup.3+ -doped fluoride fiber laser.
FIG. 2 is an energy level diagram showing a pumping method and transitions when using cross relaxation pumping for the Pr.sup.3+ fluoride upconversion laser. In the figure, one pump photon 22 creates GSA to the .sup.1 G.sub.4 (100 .mu.s) level, while another pump photon 26 boosts the Pr.sup.3+ ion from that energy level or some decayed level above the ground state to the desired 3P0 level. The excited ion 28 in this state has favorable branching to the ground state which is used as a lasing transition. This method is advantageous since the peak absorption coefficient of the Yb.sup.3+ is almost two orders of magnitude greater than the peak Pr.sup.3+ absorption at 1017 nm. This permits the realization of shorter lasers than with lasers doped strictly with Pr.sup.3+.
Known cavity designs for the Pr.sup.3+ blue laser consist of Fabry-Perot designs where dielectric mirrors are butted against cleaved or polished end facets with the pumping schemes indicated above. Pump combining has been achieved through the use of polarization beam combining. Recently, France Telecom announced a fluoride fiber laser having TiO.sub.2 -SiO.sub.2 multi-layer dielectric mirrors directly coated to the fiber endfaces (see Fiber Optics Newswire, Avelon Corp (3 Jul. 1995)).
Unfortunately, the state of the art is such that none of the presently known designs are manufacturable, either in terms of size, energy requirements, and/or cost of manufacture. It would therefore be advantageous to provide a small, low cost, low energy consumption laser that generates blue light.